Green’s Dictionary of Slang

(US) an Irish Catholic; also as adj.

1904		‘Number 1500’ Life In Sing Sing 253: Tad. An Irishman.
1905		F. Hutchison Philosophy of Johnny the Gent 22: This Tad had been used to swingin’ a pick an’ shovel all his life, an’ somebody croaked an left him a gob o’ dough.
1928		J. Callahan Man’s Grim Justice 124: Danny was one of those hard-headed tads.
1949		Monteleone Criminal Sl. (rev. edn).
1951		Green & Laurie Show Biz from Vaude to Video 571: Tad comic – Irish comedian.
1982		G.V. Higgins Patriot Game (1985) 85: Some simple-minded old tad that’s most likely been tiddly on the communion wine for about a hundred years.